If you like us, please share us on social media.
The latest UCD Hyperlibrary newsletter is now complete, check it out.

GeoWiki.png
ChemWiki: The Dynamic Chemistry E-textbook > Inorganic Chemistry > Molecular Geometry

Molecular Geometry

The specific three dimensional arrangement of atoms in molecules is referred to as molecular geometry. We also define molecular geometry as the positions of the atomic nuclei in a molecule. There are various instrumental techniques such as X-Ray crystallography and other experimental techniques which can be used to tell us where the atoms are located in a molecule. Using advanced techniques, very complicated structures for proteins, enzymes, DNA, and RNA have been determined. Molecular geometry is associated with the chemistry of vision, smell and odors, taste, drug reactions and enzyme controlled reactions to name a few.

Molecular geometry is associated with the specific orientation of bonding atoms. A careful analysis of electron distributions in orbitals will usually result in correct molecular geometry determinations. In addition, the simple writing of Lewis diagrams can also provide important clues for the determination of molecular geometry. Click on a picture to link to a page with the GIF file and a short discussion of the molecule.

Steric Number (# bonded atoms + # electron pairs)
6 5 4 3 2
AX6
ax6.gif
octahedral
AX5
ax5.gif
trigonal bipyramidal
AX4
ax4.gif 
tetrahedral
AX3
  ax3.gif
trigonal planar
AX2
nopic.gif
linear
1 lone pair of electrons
AX5E
ax5e.gif
square pyramidal
AX4E
ax4e.gif
distorted tetrahedron
AX3E
ax3e.gif
pyramidal
AX2E
ax2e.gif
nonlinear
AXE
nopic.gif
linear
2 lone pairs of electrons
AX4E2
ax4e2.gif
square planar
AX3E2
ax3e2.gif
T-shaped
AX2E2
ax2e2.gif
bent
AXE2
nopic.gif
linear
 
3 lone pairs of electrons
AX3E3
nopic.gif
T-shaped
AX2E3
ax2e3.gif
linear
AXE3
nopic.gif
linear
   
4 lone pairs
AX2E4
nopic.gif
linear
AXE4
nopic.gif
linear
     
5 lone pairs
AXE5
nopic.gif
linear
     

Valence Shell Electron Pair Repulsion (VSEPR) theory

Electron pairs around a central atom arrange themselves so that they can be as far apart as possible from each other. The valence shell is the outermost electron-occupied shell of an atom that holds the electrons involved in bonding. In a covalent bond, a pair of electrons is shared between two atoms. In a polyatomic molecule, several atoms are bonded to a central atom using two or more electron pairs. The repulsion between negatively charged electron pairs in bonds or as lone pairs causes them to spread apart as much as possible.

The idea of "electron pair repulsion can be demonstrated by tying several inflated balloons together at their necks. Each balloon represents an electron pair. The balloons will try to minimize the crowding and will spread as far apart as possible. According to VSEPR theory, molecular geometry can be predicted by starting with the electron pair geometry about the central atom and adding atoms to some or all of the electron pairs. This model produces good agreement with experimental determinations for simple molecules. With this model in mind, the molecular geometry can be determined in a systematic way.

  • Lewis diagrams provide information about what atoms are bonded to each other and the total electron pairs involved.
  • Electron pair geometry is determined from the total electron pairs.

Molecules can then be divided into two groups:

  • Group 1: Molecules with NO lone electron pairs. In this case the molecular geometry is identical to the electron pair geometry.
  • Group 2: Molecules with one or more lone electron pairs. In this case an extra step is needed to to translate from electron pair geometry to the final molecular geometry, since only the positions of bonded atoms are considered in molecular geometry.

Contributors

  • Charles Ophardt, Professor Emeritus, Elmhurst College; Virtual Chembook
  • Robyn Rindge (Class of '98) who now works for PDI Dreamworks (look for his name in the credits of Shrek2.). Robyn drew these rotating molecules using Infini-D (MetaCreations).
  • Paul Groves, chemistry teacher at South Pasadena High School and Chemmy Bear

You must to post a comment.
Last Modified
12:33, 6 Nov 2014

Tags

Creative Commons License Unless otherwise noted, content in the UC Davis ChemWiki is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 United States License. Permissions beyond the scope of this license may be available at copyright@ucdavis.edu. Questions and concerns can be directed toward Prof. Delmar Larsen (dlarsen@ucdavis.edu), Founder and Director. Terms of Use